Steel sheets are processed continuously in continuous galvanizing, continuous annealing, and tin mill lines of steel cold mills. In order to optimize the efficiency of the mills, the steel sheets are joined end to end via lap-seam welding. Specifically, the tail or trailing end of a preceding (first) coil and the head end of an incoming (second) coil are joined together at the entry end of the mill, thereby creating a continuous joined sheet that may be processed continuously in the mill at a much higher efficiency than would be realized if the sheets were individually processed. A conventional lap-seam or mash-seam welder may be used effectively for welding low carbon and high strength low alloy (“HSLA”) grade steel. The weld is formed in a single pass, in which a welding device, such as a pair of opposing electrodes mounted on a carriage, moves along overlapping portions of the HSLA grade steel to form a weld, before returning to its home position in idle mode.
Recently, there has been an increased demand for advanced high strength steels (AHSS) generally having a tensile strength greater than that of HSLA grade steel. AHSS are characterized by their high carbon equivalent, high tensile strength, and high electrical resistivity. In the automobile industry, for example, the use of AHSS and their heightened tensile strengths in a vehicle frame permits the production of automotive components with reduced weight and accompanying fuel efficiency improvements without adversely affecting the safety of the vehicle.
It is particularly advantageous and efficient to process AHSS in a continuous manner for performing operations such as continuous pickling and continuous galvanizing to meet the requirements of automotive customers. However, the application of conventional welding apparatus and operations, such as single-pass lap seam welding process, to galvanized TRansformation Induced Plasticity (TRIP) grade AHSS results in a brittle and weak weld due to martensite and oxide formation. Further, very high alloy content (high carbon equivalent) and high resistivity of AHSS makes these grades ultra-sensitive to welding parameters. Microstructure studies have shown that AHSS grade welds often undergo excessive surface heating (expulsion) and generate hot micro-cracks, porosity, and inadequate fusion when subjected to the single weld pass employed for HSLA. Performance criteria required for safe and reliable processing through the mill are generally not satisfied by the brittle welds created by conventional single-pass lap seam welding practices. Failure of the weld during mill processing may cause shut down of the line for relatively short (e.g., 1 hour) or extended (e.g., 1 day) periods, depending on the location and severity of the weld break.
To remedy this problem, a solution has been proposed in which a low carbon HSLA grade steel “stringer” coil is interposed between two AHSS (e.g., TRIP) coils so that the TRIP-to-TRIP weld is replaced by stronger and more reliable HSLA-to-TRIP and TRIP-to-HSLA welds for indirectly joining the AHSS coils together. Specifically, the tail end of the preceding HSLA coil is joined to the head end of a TRIP coil and the tail end of the TRIP coil is joined to the head end of another HSLA insert coil. The additional labor and materials required for implementation of this remedy reduces overall productivity and increases expenses. Furthermore, such a procedure requires scheduling and stocking an inventory of the necessary HSLA insert coils. Other costs include quality defects due to unstable processing conditions, and greater equipment wear and tear.
Another proposed remedy to the welding of AHSS is the application of induction heating after welding. This alternative solution requires the installation of an induction heating unit or separate station requiring capital investment and significant additional processing time to cool down the weld.